Semiconductor device and driving method thereof

ABSTRACT

A novel semiconductor device and a driving method thereof are provided. In the semiconductor device, a (volatile) node which holds data that is rewritten by arithmetic processing as appropriate and a node in which the data is stored are electrically connected through a source and a drain of a transistor whose channel is formed in an oxide semiconductor layer. The off-state current value of the transistor is extremely low. Therefore, electric charge scarcely leaks through the transistor from the latter node, and thus data can be held in the latter node even in a period during which supply of power source voltage is stopped. In the semiconductor device, a means of setting the potential of the latter node to a predetermined potential is provided. Specifically, a means of supplying a potential corresponding to &#34;1&#34; or &#34;0&#34; that is data stored in the latter node from the former node is provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device. In particular,the present invention relates to a semiconductor device to which powersupply is stopped as appropriate during its operation. Further, thepresent invention relates to a driving method of the semiconductordevice. Note that in this specification, a semiconductor device refersto a device which is capable of functioning by utilizing semiconductorcharacteristics.

2. Description of the Related Art

A semiconductor device such as a central processing unit (CPU) has avariety of configurations depending on its application. Thesemiconductor device generally has some kinds of memory devices such asa register and a cache memory as well as a main memory for storing dataor a program. A register has a function of temporarily holding data forcarrying out arithmetic processing, holding a program execution state,or the like. In addition, a cache memory is located between anarithmetic circuit and a low-speed main memory in order to reduce accessto the main memory and speed up the arithmetic processing.

A register and a cache memory need to operate at higher speed in readingor writing data than a main memory. Thus, in many cases, a flip-flop isused as a register, and a static random access memory (SRAM) is used asa cache memory.

In order to reduce power consumption, supply of power source voltage toa semiconductor device is preferably stopped as appropriate (forexample, in a period during which data is not input or output). Notethat a flip-flop, an SRAM, and the like are volatile memory devices.Therefore, in the case where supply of power source voltage to asemiconductor device is stopped, data which has been lost in a volatilememory device such as a register or a cache memory need to be restoredafter the supply of power source voltage is restarted. For example, thefollowing process is necessary: needed data is read from an auxiliarymemory device provided in the outside of the semiconductor device to benewly written to the volatile memory device in a signal processingcircuit. This leads to an operation delay of the signal processingcircuit.

In view of this, a semiconductor device in which a nonvolatile memorydevice is located in the periphery of a volatile memory device has beendeveloped. For example, Patent Document 1 discloses the followingtechnique: data held in a flip-flop or the like is stored in aferroelectric memory before supply of power source voltage is stopped,and the data stored in the ferroelectric memory is restored to theflip-flop or the like after the supply of power source voltage isrestarted.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    H10-078836

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide anovel semiconductor device and a driving method thereof. Specifically,an object is to provide a semiconductor device such as a centralprocessing unit whose power consumption can be reduced and operationdelay can be prevented.

In a semiconductor device of one embodiment of the present invention, a(volatile) node which holds data that is rewritten by arithmeticprocessing as appropriate and a node in which the data is stored areelectrically connected to each other through a source and a drain of atransistor whose channel is formed in an oxide semiconductor layer. Notethat the off-state current value of the transistor is extremely low. Forthat reason, electric charge scarcely leaks through the transistor fromthe latter node. As a result, data can be held in the latter node evenin a period during which supply of power source voltage is stopped. Notethat in this specification, stopping supply of power source voltagemeans not supplying the power source voltage (high power sourcepotential or low power source potential) or making the power sourcevoltage (a potential difference between the high power source potentialand the low power source potential) zero or substantially zero.

The semiconductor device is provided with a means of setting thepotential of the latter node to a predetermined potential. Specifically,the semiconductor device is provided with a means of supplying apotential corresponding to “1” or “0” that is data stored in the latternode from the former node.

For example, one embodiment of the present invention is a semiconductordevice including: a first node capable of holding a potentialcorresponding to 1 or 0 as data in a period during which a power sourcevoltage is supplied; a second node capable of holding the dataregardless of whether the power source voltage is supplied; a firsttransistor which is configured to control whether to electricallyconnect the first node and the second node and in which a channel isformed in an oxide semiconductor layer; a second transistor a switchingof which depends on a potential of the second node; and a means ofsupplying the potential corresponding to 1 or 0 to the second noderegardless of the data.

Another embodiment of the present invention is a driving method of thesemiconductor device, including: a first step of supplying the potentialcorresponding to 1 or 0 to the second node by the means; a second stepof turning on the first transistor after the first step to store thedata in the second node from the first node; a third step of stoppingsupply of the power source voltage after the second step; a fourth stepof restarting the supply of the power source voltage after the thirdstep; and a fifth step of restoring the data held in the first nodebefore the second step depending on an on/off state of the secondtransistor after the fourth step.

Supply of power source voltage to a semiconductor device of oneembodiment of the present invention is stopped as appropriate, so thatthe power consumption of the semiconductor device can be reduced.Further, data is stored before the supply of power source voltage isstopped and is restored after the supply of power source voltage isrestarted, so that an operation delay can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a configuration example of a semiconductor device,and FIG. 1B is a flow chart illustrating an example of a driving methodof the semiconductor device.

FIG. 2A illustrates a configuration example of a semiconductor device,and FIG. 2B is a timing diagram showing an example of potentials andsignals supplied to the semiconductor device.

FIG. 3 illustrates a configuration example of a semiconductor device.

FIG. 4A illustrates a configuration example of a semiconductor device,and FIG. 4B is a timing diagram showing an example of potentials andsignals supplied to the semiconductor device.

FIG. 5A illustrates a configuration example of an arithmetic portion,and FIG. 5B is a timing diagram showing an example of potentials andsignals supplied to the arithmetic portion.

FIG. 6 illustrates a structure example of a transistor.

FIG. 7 illustrates a structure example of a semiconductor device.

FIGS. 8A to 8F each illustrate a specific example of an electronicdevice.

FIG. 9 illustrates a configuration example of a semiconductor device.

FIG. 10 illustrates a configuration example of a semiconductor device.

FIG. 11 shows measurement results of time required for data writing.

FIG. 12 shows measurement results of time required for data writing.

FIG. 13 shows a layout of a processor.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below in detail.Note that the present invention is not limited to the description below,and a variety of changes can be made without departing from the spiritand scope of the present invention. Therefore, the present inventionshould not be construed as being limited to the description below.

<Configuration Example of Semiconductor Device and Example of DrivingMethod Thereof>

FIG. 1A illustrates the configuration example of a semiconductor deviceof one embodiment of the present invention, and FIG. 1B is a flow chartillustrating an example of a driving method in data storing and datarestoring of the semiconductor device illustrated in FIG. 1A.

The semiconductor device illustrated in FIG. 1A includes an arithmeticportion 1 that performs arithmetic processing and a data storing portion2 where data held in the arithmetic portion 1 is stored.

The arithmetic portion 1 is supplied with a power source potential V1and a power source potential V2. The arithmetic portion 1 can operateusing a difference between the power source potentials V1 and V2 as apower source voltage. Note that here, the power source potential V1 is ahigh power source potential VDD or a low power source potential VSS, andthe power source potential V2 is the low power source potential VSS. Thearithmetic portion 1 has a node (Node_1) in which a potentialcorresponding to “1” or “0” can be held as data in a period during whichthe power source voltage is supplied.

The data storing portion 2 includes a transistor 21 a gate of which iselectrically connected to a wiring supplying a signal OS_WE, one of asource and a drain of which is electrically connected to the node(Node_1) of the arithmetic portion 1, and the other of the source andthe drain of which is electrically connected to a node (Node_2); acapacitor 22 one electrode of which is electrically connected to thenode (Node_2) and the other electrode of which is electrically connectedto a wiring supplying the power source potential V2; a transistor 23 agate of which is electrically connected to the node (Node_2), one of asource and a drain of which is electrically to the wiring supplying thepower source potential V2, and the other of the source and the drain ofwhich is electrically connected to the arithmetic portion 1; and a means24 capable of supplying a potential corresponding to “1” or “0” that isdata held in the node (Node_1) in the arithmetic portion 1 to the node(Node_2). Note that the capacitor 22 is not necessarily provided in thedata storing portion 2 illustrated in FIG. 1A.

Here, the potential of the signal OS_WE is higher than the sum of thethreshold voltage of the transistor 21 and the high power sourcepotential VDD, and the potential of the signal OS_WE is the low powersource potential VSS (this case is referred to as a former case).Alternatively, the potential of the signal OS_WE can be the high powersource potential VDD, and the potential of the signal OS_WE can be thelow power source potential VSS (this case is referred to as a lattercase). In the latter case, the power source potential needed for drivingthe semiconductor device can be reduced as compared with the formercase. On the other hand, in the former case, the high power sourcepotential VDD can be supplied to the node (Node_2), so that data storingdescribed later can be performed surely.

Note that the transistor 21 is a transistor whose channel is formed inan oxide semiconductor layer. A path for supplying electric charge tothe node (Node_2) is only a path through the source and the drain of thetransistor whose channel is formed in the oxide semiconductor layer. Thetransistor whose channel is formed in the oxide semiconductor layer hasan extremely low off-state current value. Therefore, in a period duringwhich the transistor is off, the potential of the node (Node_2) can bekept substantially constant. For this reason, data can be held in thenode (Node_2) regardless of whether the power source voltage issupplied. In other words, data held in the node (Node_1) in thearithmetic portion 1 can be stored in the node (Node_2).

The transistor 23 can be formed using any of a variety of semiconductormaterials. For example, a material such as silicon or germanium can beused. Alternatively, it is possible to use a compound semiconductor oran oxide semiconductor. Note that as the transistor 23, a transistorwhose mobility is high (for example, a transistor in which a channel isformed in single crystal silicon) is preferably used.

Data which has been held in the node (Node_1) in the arithmetic portion1 and then lost by stop of supply of power source voltage can berestored in the arithmetic portion 1 depending on the on/off state ofthe transistor 23.

Hereinafter, an example of a driving method of the semiconductor deviceillustrated in FIG. 1A will be described with reference to FIG. 1B.

In the semiconductor device illustrated in FIG. 1A, the potential of thenode (Node_2) can be initialized (S1). Specifically, a potentialcorresponding to “1” or “0” that is data held in the node (Node_1) inthe arithmetic portion 1 can be supplied to the node (Node_2) by themeans 24.

Next, data held in the node (Node_1) in the arithmetic portion 1 can bestored in the node (Node_2) in the data storing portion 2 (S2).Specifically, the transistor 21 is turned on, whereby the data held inthe node (Node_1) is written to the node (Node_2). Since the potentialcorresponding to “1” or “0” is supplied to the node (Node_2), thepotential of the node (Node_2) is not changed by the storing in somecases.

Next, supply of power source voltage to the semiconductor device can bestopped (S3). Specifically, the power source potential V1 is set to thelow power source potential VSS.

Next, the supply of power source voltage to the semiconductor device canbe restarted (S4). Specifically, the power source potential V1 is set tothe high power source potential VDD.

Then, the data that has been held in the node (Node_1) in the arithmeticportion 1 can be restored (S5). Specifically, the data in the node(Node_1) is restored depending on the on/off state of the transistor 23.

As described above, supply of power source voltage to the semiconductordevice illustrated in FIG. 1A is stopped, so that the power consumptionof the semiconductor device illustrated in FIG. 1A can be reduced.Further, the data that has been held in the node (Node_1) is restoredafter the supply of power source voltage is restarted, so that anoperation delay can be prevented.

In the semiconductor device illustrated in FIG. 1A, the potential of thenode (Node_2) is initialized before the data is stored. In this case,the time required for storing the data can be short as compared with thecase where the initialization is not performed. This point is describedbelow in detail.

Storing data in the semiconductor device illustrated in FIG. 1A meansthat the potential of the node (Node_2) is changed into a potentialcorresponding to data held in the node (Node_1). That is, the potentialof the node (Node_2) is rewritten depending on the data storing in thefollowing manner: the potential corresponding to “1” is rewritten intothe potential corresponding to “0”, or the potential corresponding to“0” is rewritten into the potential corresponding to “1”.

In the semiconductor device illustrated in FIG. 1A, the time requiredfor rewriting the potential corresponding to “0” into the potentialcorresponding to “1” and the time required for rewriting the potentialcorresponding to “1” into the potential corresponding to “0” are notnecessarily the same. Specifically, in the semiconductor deviceillustrated in FIG. 1A, data held in the node (Node_2) is identified onthe basis of the on/off state of the transistor 23. That is, the dataheld in the node (Node_2) is identified on the basis of not only thepotential of the node (Node_2) but also the threshold voltage of thetransistor 23. For such a reason, both of the times depend on thethreshold voltage of the transistor 23. For example, in the case wherethe threshold voltage of the transistor 23 is a value nearer to thepotential corresponding to “0” than to the potential corresponding to“1”, the time required for rewriting the potential corresponding to “1”into the potential corresponding to “0” is longer than the time requiredfor rewriting the potential corresponding to “0” into the potentialcorresponding to “1”. Further, in the case where there is a possibilitythat both the potential corresponding to “0” and the potentialcorresponding to “1” need to be rewritten in storing data, the longertime (the time required for rewriting the potential corresponding to “1”into the potential corresponding to “0”) is set as the time required forstoring data. This is for surely storing data.

In the semiconductor device illustrated in FIG. 1A, the potential of thenode (Node_2) is set in advance so as to correspond to “1” or “0”. Thatis, data which takes a longer time to be stored can be supplied to thenode (Node_2) in advance. Accordingly, in the semiconductor deviceillustrated in FIG. 1A, the shorter time required for storing data canbe set as the time required for storing data. That is, the time requiredfor storing data can be shortened. As a result, in the semiconductordevice illustrated in FIG. 1A, an operation delay can be prevented.

<Specific Example 1 of Means 24>

FIG. 2A illustrates the configuration example of a semiconductor devicein which the means 24 is specifically shown. FIG. 2B is an example of atiming diagram of the power source potentials V1 and V2 and signalsOS_WE and OS_WE2 supplied to the semiconductor device illustrated inFIG. 2A.

The semiconductor device illustrated in FIG. 2A includes, as the means24, an analog switch 25 controlling whether to electrically connect thenode (Node_1) in the arithmetic portion 1 and one of a source and adrain of the transistor 21 and an analog switch 26 controlling whetherto electrically connect a wiring supplying the power source potential V2and the one of the source and the drain of the transistor 21. Note thatthe analog switches 25 and 26 operate in an inverse manner with thesignal OS_WE2 and an inverted signal of the signal OS_WE2 (an outputsignal of an inverter 27). That is, in the semiconductor deviceillustrated in FIG. 2A, a switch is formed of the analog switches 25 and26 and the inverter 27. Here, the potential of the signal OS_WE2 is thehigh power source potential VDD or the low power source potential VSS.

In the semiconductor device illustrated in FIG. 2A, the potential of thenode (Node_2) is initialized in periods (Periods T1 and T3 shown in FIG.2B) prior to periods during which data is stored (Periods T2 and T4shown in FIG. 2B). Specifically, the node (Node_2) is supplied with thepower source potential V2 (the low power source potential VSS) throughthe analog switch 26 and the source and the drain of the transistor 21.Note that in the semiconductor device illustrated in FIG. 2A, Periods Taand Tc shown in FIG. 2B are periods during which arithmetic processingcan be performed in the arithmetic portion 1, and Periods Tb and Td areperiods during which supply of power source voltage is stopped.

Here, the length of the periods during which the potential of the node(Node_2) is initialized (Periods T1 and T3 shown in FIG. 2B) ispreferably longer than the length of the periods during which data isstored (Periods T2 and T4 shown in FIG. 2B). The reason for this is asfollows: one of the potential corresponding to “0” into which thepotential corresponding to “1” is rewritten and the potentialcorresponding to “1” into which the potential corresponding to “0”,which takes a longer time to be stored, can be supplied to the node(Node_2) in advance (in Periods T1 and T3).

Note that the analog switch 26 included in the semiconductor deviceillustrated in FIG. 2A may be replaced with a transistor 28 asillustrated in FIG. 3. Note that the switching of the transistor 28 iscontrolled with the signal OS_WE2.

<Specific Example 2 of Means 24>

FIG. 4A illustrates the configuration example of a semiconductor devicein which the means 24 different from that in the semiconductor deviceillustrated in FIG. 2A is specifically shown. FIG. 4B is an example of atiming diagram of the power source potentials V1 and V2, the signalOS_WE, and a signal OS_WE3 supplied to the semiconductor deviceillustrated in FIG. 4A.

The semiconductor device illustrated in FIG. 4A includes, as the means24, a transistor 29 controlling whether to electrically connect a wiringsupplying the power source potential V2 and the node (Node_2). Note thatthe switching of the transistor 29 is controlled with the signal OS_WE3.The transistor 29 is a transistor whose channel is formed in an oxidesemiconductor layer. Here, the potential of the signal OS_WE3 is thehigh power source potential VDD or the low power source potential VSS.

In the semiconductor device illustrated in FIG. 4A, the potential of thenode (Node_2) is initialized in periods (Periods t1 and t3 shown in FIG.4B) prior to periods during which data is stored (Periods t2 and t4shown in FIG. 4B). Specifically, when the transistor 29 is turned on,the node (Node_2) is supplied with the power source potential V2 (thelow power source potential VSS). Note that in the semiconductor deviceillustrated in FIG. 4A, Periods to and tc shown in FIG. 4B are periodsduring which arithmetic processing can be performed in the arithmeticportion 1, and Periods tb and td are periods during which supply ofpower source voltage is stopped.

Here, the length of the periods during which the potential of the node(Node_2) is initialized (Periods t1 and t3 shown in FIG. 4B) ispreferably longer than the length of the periods during which data isstored (Periods t2 and t4 shown in FIG. 4B).

<Example of Data Restoring in Arithmetic Portion 1>

FIG. 5A illustrates the configuration example of the arithmetic portion1 included in the semiconductor devices illustrated in FIGS. 1A and 1B,FIGS. 2A and 2B, FIG. 3, and FIGS. 4A and 4B. FIG. 5B is an example of atiming diagram of the power source potentials V1 and V2, a signal CK, asignal Reset, and a signal OS_RD supplied to the semiconductor deviceillustrated in FIG. 5A. Here, the potential of each of the signal CK,the signal Reset, and the signal OS_RD is the high power sourcepotential VDD or the low power source potential VSS.

The arithmetic portion 1 illustrated in FIG. 5A includes an inverter 10outputting a signal to the node (Node_1); a NAND gate 11 to which thesignal Reset is input as one input and the potential of the node(Node_1) is input as the other input; an analog switch 12 controllingwhether to electrically connect an input terminal (In) of the arithmeticportion 1 and the node (Node_1); an analog switch 13 controlling whetherto electrically connect an output terminal of the NAND gate 11 and eachof an output terminal (Out) of the arithmetic portion 1 and an inputterminal of the inverter 10; and an analog switch 14 controlling whetherto electrically connect a source or a drain of the transistor 23included in the data storing portion 2 and each of the output terminal(Out) of the arithmetic portion 1 and the input terminal of the inverter10.

Note that the analog switch 12 operates in response to the signal CK andan inverted signal of the signal CK (output signal of an inverter 15).The analog switches 13 and 14 operate in an inverse manner with thesignal OS_RD and an inverted signal of the signal OS_RD (an outputsignal of an inverter 16). That is, in the semiconductor deviceillustrated in FIG. 5A, a switch is formed of the analog switches 13 and14 and the inverter 16.

In the arithmetic portion 1 illustrated in FIG. 5A, after supply ofpower source voltage is restarted, the potential of a signal output fromthe NAND gate 11 is the high power source potential VDD in a periodduring which the potential of the signal CK is the high power sourcepotential VDD and the potential of the signal Reset is the low powersource potential VSS (in Period T_Reset shown in FIG. 5B). Since thepotential of the signal OS_RD is the low power source potential VSS inPeriod T_Reset, the potential of a signal output from the outputterminal (Out) of the arithmetic portion 1 is the high power sourcepotential VDD and the potential of the node (Node_1) is the low powersource potential VSS. In a period during which the potential of thesignal OS_RD is the high power source potential VDD after the potentialof the signal Reset is changed into the high power source potential VDD(in Period T_RD shown in FIG. 5B), data that has been held in the node(Node_1) can be restored. Specifically, data that has been held in thenode (Node_1) can be restored in such a manner that the potential of thenode (Node_1) is raised to the high power source potential VDD when thetransistor 23 is on, and the potential of the node (Node_1) is kept atthe low power source potential VSS when the transistor 23 is off.

<Example of Transistor Whose Channel is Formed in Oxide SemiconductorLayer>

FIG. 6 illustrates an example of a structure of a transistor whosechannel is formed in an oxide semiconductor layer. The transistorillustrated in FIG. 6 includes an oxide semiconductor layer 31 providedover a layer 30 having an insulating surface, a conductive layer 32 incontact with one end of the oxide semiconductor layer 31, a conductivelayer 33 in contact with the other end of the oxide semiconductor layer31, an insulating layer 34 provided over the oxide semiconductor layer31 and the conductive layers 32 and 33, and a conductive layer 35provided over the insulating layer 34. Note that in the transistorillustrated in FIG. 6, the conductive layers 32 and 33 function as asource and a drain, the insulating layer 34 functions as a gateinsulating film, and the conductive layer 35 functions as a gate.

<1. Specific Example of Oxide Semiconductor Layer 31> <(1) OxideSemiconductor Material>

A film containing at least indium can be used as the oxide semiconductorlayer 31. In particular, a film containing indium and zinc is preferablyused. As a stabilizer for reducing variations in electriccharacteristics of the transistor, a film containing gallium (Ga) inaddition to indium and zinc is preferably used.

Alternatively, a film which contains, as a stabilizer, one or more oftin, hafnium, aluminum, zirconium, and lanthanoid such as lanthanum,cerium, praseodymium, neodymium, samarium, europium, gadolinium,terbium, dysprosium, holmium, erbium, thulium, ytterbium, or lutetiumcan be used as the oxide semiconductor layer 31.

As the oxide semiconductor layer 31, for example, a film of any of thefollowing oxides can be used: indium oxide; a two-component metal oxidesuch as an In—Zn-based oxide, an In—Mg-based oxide, or an In—Ga-basedoxide; a three-component metal oxide such as an In—Ga—Zn-based oxide, anIn—Al—Zn-based oxide, an In—Sn—Zn-based oxide, an In—Hf—Zn-based oxide,an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-basedoxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, anIn—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide,an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-basedoxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, or anIn—Lu—Zn-based oxide; and a four-component metal oxide such as anIn—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, anIn—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, anIn—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide.

Note that here, for example, an “In—Ga—Zn-based oxide” means an oxidecontaining In, Ga, and Zn as its main component and there is noparticular limitation on the ratio of In:Ga:Zn. The In—Ga—Zn-based oxidemay contain a metal element other than In, Ga, and Zn.

Note that part of oxygen included in the oxide semiconductor layer 31may be substituted with nitrogen.

<(2) Crystal Structure of Oxide Semiconductor>

For the oxide semiconductor layer 31, a film having a single crystalstate, a polycrystalline (also referred to as polycrystal) state, anamorphous state, or the like can be used. In addition, a CAAC-OS (c-axisaligned crystalline oxide semiconductor) film can be used for the oxidesemiconductor layer 31. The CAAC-OS film is described in detail below.

The CAAC-OS film is one of oxide semiconductor films including aplurality of c-axis aligned crystal parts.

In a transmission electron microscope (TEM) image of the CAAC-OS film, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflected by a surface over which theCAAC-OS film is formed (hereinafter, a surface over which the CAAC-OSfilm is formed is referred to as a formation surface) or a top surfaceof the CAAC-OS film, and is arranged in parallel to the formationsurface or the top surface of the CAAC-OS film.

On the other hand, according to the TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface (plan TEM image), metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

From the results of the cross-sectional TEM image and the plan TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

Most of the crystal parts included in the CAAC-OS film each fit inside acube whose one side is less than 100 nm. Thus, there is a case where acrystal part included in the CAAC-OS film fits inside a cube whose oneside is less than 10 nm, less than 5 nm, or less than 3 nm. Note thatwhen a plurality of crystal parts included in the CAAC-OS film areconnected to each other, one large crystal region is formed in somecases. For example, a crystal region with an area of 2500 nm² or more, 5μm² or more, or 1000 μm² or more is observed in some cases in the planTEM image.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in a direction substantiallyperpendicular to the c-axis, a peak appears frequently when 2θ is around56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal.Here, analysis (φ scan) is performed under conditions where the sampleis rotated around a normal vector of a sample surface as an axis (φaxis) with 2θ fixed at around 56°. In the case where the sample is asingle-crystal oxide semiconductor film of InGaZnO₄, six peaks appear.The six peaks are derived from crystal planes equivalent to the (110)plane. On the other hand, in the case of a CAAC-OS film, a peak is notclearly observed even when φ scan is performed with 2θ fixed at around56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned in adirection parallel to a normal vector of a formation surface or a normalvector of a top surface. Thus, for example, in the case where a shape ofthe CAAC-OS film is changed by etching or the like, the c-axis might notbe necessarily parallel to a normal vector of a formation surface or anormal vector of a top surface of the CAAC-OS film.

In the CAAC-OS film, distribution of c-axis aligned crystal parts is notnecessarily uniform. For example, in the case where crystal growthleading to the crystal parts of the CAAC-OS film occurs from thevicinity of the top surface of the film, the proportion of the c-axisaligned crystal parts in the vicinity of the top surface is higher thanthat in the vicinity of the formation surface in some cases. Further,when an impurity is added to the CAAC-OS film, a region to which theimpurity is added is altered, and the proportion of the c-axis alignedcrystal parts in the CAAC-OS film varies depending on regions, in somecases.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ do not appear at around36°.

The CAAC-OS film is an oxide semiconductor film having a low impurityconcentration. The impurity means an element other than main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element (e.g., silicon)having higher strength of bonding to oxygen than a metal elementincluded in the oxide semiconductor film takes oxygen away in the oxidesemiconductor film to disrupt the atomic arrangement in the oxidesemiconductor film, which causes a lowering of the crystallinity of theoxide semiconductor film. A heavy metal such as iron or nickel, argon,carbon dioxide, or the like has a large atomic radius (or molecularradius), and thus disrupts the atomic arrangement in the oxidesemiconductor film when included in the oxide semiconductor film, whichcauses a lowering of the crystallinity of the oxide semiconductor film.Note that the impurity included in the oxide semiconductor film servesas a carrier trap or a carrier generation source in some cases.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. For example, oxygen vacancies in the oxide semiconductorfilm serve as carrier traps or serve as carrier generation sources whenhydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (few oxygen vacancies) is referred to as “highly purifiedintrinsic” or “substantially highly purified intrinsic”. A highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor film has few carrier generation sources, and thus has alow carrier density. Thus, a transistor including the oxidesemiconductor film rarely has a negative threshold voltage (rarely hasnormally-on characteristics). A highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has fewcarrier traps. Thus, the transistor including the oxide semiconductorfilm has a small variation in electric characteristics and accordinglyhas high reliability. Charges trapped by the carrier traps in the oxidesemiconductor film take a long time to be released and may behave likefixed charges. Thus, the transistor including the oxide semiconductorfilm with a high impurity concentration and a high density of defectstates has unstable electric characteristics in some cases.

In a transistor using the CAAC-OS film, change in electriccharacteristics due to irradiation with visible light or ultravioletlight is small.

Note that the oxide semiconductor layer 31 may be a stacked filmincluding two or more films of an amorphous oxide semiconductor film, amicrocrystalline oxide semiconductor film, and a CAAC-OS film, forexample.

<(3) Layer Structure of Oxide Semiconductor>

For the oxide semiconductor layer 31, not only a single-layer oxidesemiconductor film but also a layer formed of a stack having pluralkinds of oxide semiconductor films may be used. For example, a layerincluding at least two of an amorphous oxide semiconductor film, apolycrystalline oxide semiconductor film, and a CAAC-OS film can be usedfor the oxide semiconductor layer 31.

Alternatively, a layer formed of a stack of oxide semiconductor filmshaving different compositions can be used for the oxide semiconductorlayer 31. Specifically, a layer including a first oxide semiconductorfilm (hereinafter also referred to as an upper layer) which is providedon the insulating layer 34 side and a second oxide semiconductor film(hereinafter also referred to as a lower layer) which is provided on thelayer 30 side and has a different composition from the first oxidesemiconductor film can be used as the oxide semiconductor layer 31.

<2. Specific Example of Conductive Layers 32 and 33>

For the conductive layer 32 and the conductive layer 33, a filmcontaining an element selected from aluminum, copper, titanium,tantalum, tungsten, molybdenum, chromium, neodymium, and scandium, afilm of an alloy containing any of these elements, a film of a nitridecontaining any of these elements, or the like can be used.Alternatively, each of the conductive layer 32 and the conductive layer33 can be formed with a stack of these films.

<3. Specific Example of Insulating Layer 34>

For the insulating layer 34, an inorganic insulating material film suchas a silicon oxide film, a silicon nitride film, a silicon oxynitridefilm, a silicon nitride oxide film, an aluminum oxide film, an aluminumoxynitride film, or a gallium oxide film can be used. Alternatively, astack of layers of these materials can also be used. In particular, analuminum oxide film is preferably used for the insulating layer 34. Thealuminum oxide film has a high shielding (blocking) effect of preventingpenetration of both oxygen and an impurity such as hydrogen. Therefore,when the layer including an aluminum oxide film is used as theinsulating layer 34, it is possible to prevent release of oxygen fromthe oxide semiconductor layer 31 and entry of an impurity such ashydrogen to the oxide semiconductor layer 31.

The insulating layer 34 can be formed using a film including a hafniumoxide film, an yttrium oxide film, a hafnium silicate (HfSi_(x)O_(y)(x>0, y>0)) film, a hafnium silicate film to which nitrogen is added, ahafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)) film, a lanthanum oxidefilm (i.e., a film formed of what is called a high-k material), or thelike. By using such a film, gate leakage current can be reduced.

<4. Specific Example of Conductive Layer 35>

For the conductive layer 35, a film containing an element selected fromaluminum, copper, titanium, tantalum, tungsten, molybdenum, chromium,neodymium, and scandium or a film of an alloy containing any of theseelements as its component can be used. Alternatively, a metal oxidecontaining nitrogen, specifically, an In—Ga—Zn—O film containingnitrogen, an In—Sn—O film containing nitrogen, an In—Ga—O filmcontaining nitrogen, an In—Zn—O film containing nitrogen, a Sn—O filmcontaining nitrogen, an In—O film containing nitrogen, or a metalnitride (e.g., InN or SnN) film can be used for the conductive layer 35.Such a nitride film has a work function of 5 eV (electron volts) orhigher, preferably 5.5 eV (electron volts) or higher, which enables thethreshold voltage of the transistor to be positive when such a nitridefilm is used as the gate, so that what is called a normally-offswitching element can be achieved. Alternatively, the conductive layer35 can be formed with a stack of these films.

<5. Supplementary Note>

In the transistor illustrated in FIG. 6, it is preferred to suppressentry of impurities into the oxide semiconductor layer 31 and release ofconstituent element of the oxide semiconductor layer 31. This is becausethe electric characteristics of the transistor are changed when suchphenomena occur. As a means of suppressing these phenomena, insulatinglayers having a high blocking effect are provided above and below thetransistor (between the layer 30 having an insulating surface and thetransistor, and over the insulating layer 34 and the conductive layer35). For the insulating layers, for example, an inorganic insulatingmaterial film such as a silicon oxide film, a silicon nitride film, asilicon oxynitride film, a silicon nitride oxide film, an aluminum oxidefilm, an aluminum oxynitride film, or a gallium oxide film can be used.Alternatively, a stack of layers of these materials can also be used.

Example 1

FIG. 9 illustrates a configuration example of a semiconductor device ofone embodiment of the present invention.

The semiconductor device illustrated in FIG. 9 includes the arithmeticportion 1 performing arithmetic processing and the data storing portion2 in which data held in the arithmetic portion 1 is stored. The datastoring portion 2 includes an inverter 227, an inverter 228, an analogswitch 229, an analog switch 230, a transistor 231, a transistor 232, atransistor 233, a transistor 234, a capacitor 235, a capacitor 236, andan inverter 237.

The signal OS_RD and the inverted signal of the signal OS_RD aresupplied to the analog switch 229 and the analog switch 230 through theinverter 227 and the inverter 228. Switching of the analog switch 229and switching of the analog switch 230 are controlled with the signalOS_RD and the inverted signal of the signal OS_RD. Switching of thetransistor 231 is controlled with the signal OS_WE that is input to agate of the transistor 231. When the analog switch 229 and thetransistor 231 are on, a potential corresponding to “1” or “0” that isdata held in the node (Node_1) is supplied to the capacitor 235 and agate of the transistor 232 through the analog switch 229 and thetransistor 231. When the gate of the transistor 232 corresponds to thenode (Node_2), switching of the transistor 232 is controlled with apotential which corresponds to “1” or “0” and is supplied to the node(Node_2).

When the transistor 231 is off, the potential corresponding to “1” or“0” is held in the node (Node_2). It is preferable that the off-statecurrent of the transistor 231 be extremely small. The reason for this isthat leakage of electric charge through the transistor 231 from the node(Node_2) can be prevented when the off-state current of the transistor231 is small.

One of a source and a drain of the transistor 232 is connected to a nodeto which the power source potential V2 is supplied. The transistor 233has a function of controlling connection between the other of the sourceand the drain of the transistor 232 and an input terminal of theinverter 237. The transistor 234 has a function of controllingconnection between a node to which the power source potential V1 issupplied and the input terminal of the inverter 237.

The capacitor 236 has a function of holding the potential of a node(Node_3) connecting one of a source and a drain of the transistor 233and one of a source and a drain of the transistor 234.

In the data storing portion 2, the potential of the node (Node_3) can bereset in such a manner that the transistor 233 is turned off, thetransistor 234 is turned on, and the power source potential V1 issupplied to the node (Node_3).

In the data storing portion 2, the potential of the node (Node_3) canreflect data held in the node (Node_2) in such a manner that thetransistor 233 is turned on and the transistor 234 is turned off.Specifically, the power source potential V2 is supplied to the node(Node_3) through the transistor 232 when the transistor 232 is on, andthe power source potential V1 is held in the node (Node_3) when thetransistor 232 is off. In this manner, the potential of the node(Node_3) reflects data held in the node (Node_2) (data input from thearithmetic portion 1).

An output signal of the inverter 237 is supplied to the arithmeticportion 1 through the analog switch 230.

With the above structure, in one embodiment of the present invention,the operation of the data storing portion 2 is controlled in thesemiconductor device; thus, data in the arithmetic portion 1 can bestored in the data storing portion 2 and data in the data storingportion 2 can be restored to the arithmetic portion 1.

Example 2

FIG. 10 illustrates a configuration example of a semiconductor device ofone embodiment of the present invention.

The semiconductor device illustrated in FIG. 10 includes the arithmeticportion 1 performing arithmetic processing and the data storing portion2 in which data held in the arithmetic portion 1 is stored. The datastoring portion 2 includes a transistor 203, a transistor 204, acapacitor 205, an analog switch 206, a transistor 207, an inverter 208,and an inverter 209.

Switching of the transistor 203 is controlled with the signal OS_WE.Switching of the analog switch 206 is controlled with the signal OS_WE2.Specifically, in FIG. 10, switching of the analog switch 206 iscontrolled with an output signal of the inverter 208 (the invertedsignal of the signal OS_WE2) and an output signal of the inverter 209(an inverted signal of the output signal of the inverter 208). Switchingof the transistor 207 is controlled with the output signal of theinverter 208 (the inverted signal of the signal OS_WE2).

When the analog switch 206 and the transistor 203 are on, a potentialcorresponding to “1” or “0” that is data held in the node (Node_1) issupplied to the node (Node_2). When the transistor 203 and thetransistor 207 are on, the power source potential V2 is supplied to thenode (Node_2).

Switching of the transistor 204 is controlled with the potential of thenode (Node_2). When the transistor 204 is on, the power source potentialV2 is supplied to the arithmetic portion 1 through the transistor 204.When the transistor 203 is off, the capacitor 205 has a function ofholding the potential of the node (Node_2).

It is preferable that the off-state current of the transistor 203 beextremely small. The reason for this is that the amount of electriccharge leaking from the node (Node_2) can be reduced and thus the lengthof a period during which data is stored in the data storing portion 2can be made long when the off-state current of the transistor 203 issmall.

Example 3

FIG. 11 shows the measurement results of the time required for writingdata to the data storing portion 2 in the semiconductor deviceillustrated in FIG. 9. FIG. 12 shows the measurement results of the timerequired for writing data to the data storing portion 2 in thesemiconductor device illustrated in FIG. 10. In FIG. 11, a period oftime T during which the analog switch 229 and the transistor 231illustrated in FIG. 9 are on is set to 200 ns (nanoseconds), 400 ns, 600ns, 800 ns, 1000 ns, 1200 ns, 1400 ns, 1600 ns, 1800 ns, and 2000 ns.Among them, the period of time T at which written data agrees with readdata and which is the shortest is set as the time required for writingdata. Like in FIG. 11, in FIG. 12, a period of time T during which theanalog switch 206 and the transistor 203 illustrated in FIG. 10 are onis set to 200 ns, 400 ns, 600 ns, 800 ns, 1000 ns, 1200 ns, 1400 ns,1600 ns, 1800 ns, and 2000 ns. Among them, the period of time T at whichwritten data agrees with read data and which is the shortest is set asthe time required for writing data.

The time required for writing data in the case where a high-levelpotential (High) is written to the node (Node_2) and the time requiredfor writing data in the case where a low-level potential (Low) iswritten to the node (Node_2) were measured. In this measurement, a powersource voltage that is a difference between the power source potentialV1 and the power source potential V2 was set to 1.6 V, 1.8 V, 2.0 V, 2.2V, and 2.4 V.

As shown in FIG. 11, the time required for writing data was 1600 ns at apower source voltage of 2.0 V in the semiconductor device illustrated inFIG. 9. On the other hand, as shown in FIG. 12, the time required forwriting data was 400 ns at a power source voltage of 2.0 V in thesemiconductor device illustrated in FIG. 10. These results indicate thatthe time required for writing data at a power source voltage of 2.0 V inthe semiconductor device illustrated in FIG. 10 is a quarter of that inthe semiconductor device illustrated in FIG. 9.

Further, in the semiconductor device illustrated in FIG. 10, the timerequired for writing data was not changed at room temperature even inthe case where data was rewritten 10¹¹ times.

Example 4

FIG. 13 shows the layout of a processor including a memory device inwhich 255 semiconductor devices illustrated in FIG. 10 are connected.The processor corresponds to one mode of a semiconductor device of oneembodiment of the present invention.

In the processor shown in FIG. 13, in the case where an applicationhaving a power-source-voltage shut-off time of several tens ofmicroseconds or more is executed, the power-source-voltage shut-off timeduring which the overhead power consumption and power cut bypower-source-voltage shut-off becomes equal in amount, that is, a breakeven time (BET) was 25.3 microseconds at a power source voltage of 2.0 Vand a driving frequency of a CPU of 25 MHz. From the above, a reductionin the power consumption of the processor can be expected when anapplication having a power-source-voltage shut-off time of several tensof microseconds or more is executed.

Example 5

In this example, referring to FIG. 7, description is given of examplesof a structure and a manufacturing method of a semiconductor deviceincluding a transistor 902 whose channel is formed in an oxidesemiconductor layer and a transistor 901 whose channel is formed in asingle crystal silicon wafer. Note that the transistor 902 can be usedas the transistor 21 illustrated in FIG. 1A, FIG. 2A, FIG. 3, and FIG.4A, and the transistor 901 can be used as the transistor 23 illustratedin FIG. 1A, FIG. 2A, FIG. 3, and FIG. 4A, for example.

Note that a semiconductor material such as germanium, silicon germanium,or single crystal silicon carbide as well as silicon may be used for thetransistor 901. A transistor including silicon can be formed using asilicon thin film formed by an SOI method or a silicon thin film formedby vapor deposition, for example; in this case, a glass substrate formedby a fusion process or a float process, a quartz substrate, asemiconductor substrate, a ceramic substrate, or the like can be used asa substrate. In the case where a glass substrate is used and thetemperature of heat treatment to be performed later is high, it ispreferable to use a glass substrate with a strain point of 730° C. orhigher.

In the semiconductor device illustrated in FIG. 7, the transistor 901using a single crystal silicon wafer is formed, and the transistor 902using an oxide semiconductor is formed above the transistor 901. Inother words, the semiconductor device described in this example is asemiconductor device that has a three-dimensional layered structure inwhich a silicon wafer is used as a substrate and a transistor layer isprovided above the silicon wafer. Moreover, the semiconductor device inthis example is a hybrid semiconductor device including a transistor inwhich silicon is used for a channel formation region and a transistor inwhich an oxide semiconductor is used for a channel formation region.

The transistor 901 formed using a substrate 900 containing asemiconductor material can be either an n-channel transistor (nMOSFET)or a p-channel transistor (pMOSFET). In the example illustrated in FIG.7, the transistor 901 is electrically isolated from other elements by ashallow trench isolation (STI) 905. The use of the STI 905 can reducegeneration of a bird's beak, which may be caused by a LOCOS elementisolation method, in an element isolation region and can reduce the sizeof the element isolation region. On the other hand, in a semiconductordevice that is not required to be structurally miniaturized ordownsized, the STI 905 is not necessarily formed and an elementisolation means such as LOCOS can be used. In the substrate 900 wherethe transistor 901 is formed, a well 904 to which an impurity impartingconductivity, such as boron, phosphorus, or arsenic, is added is formed.

The transistor 901 in FIG. 7 includes a channel formation region in thesubstrate 900, impurity regions 906 (also referred to as a source regionand a drain region) provided such that the channel formation region isplaced therebetween, a gate insulating film 907 over the channelformation region, and a gate electrode layer 908 provided over the gateinsulating film 907 to overlap with the channel formation region. Thegate electrode layer 908 can have a stacked structure of a gateelectrode layer including a first material for increasing processingaccuracy and a gate electrode layer including a second material fordecreasing the resistance as a wiring. For example, the gate electrodelayer can have a stacked structure of crystalline silicon to which animpurity imparting conductivity, such as phosphorus, is added and nickelsilicide. Note that the structure is not limited to this, and materials,the number of stacked layers, the shape, or the like can be adjusted asappropriate depending on required specifications.

Note that the transistor 901 illustrated in FIG. 7 may be a fin-typetransistor. In a fin-type structure, part of a semiconductor substrateis processed into a plate-shaped protrusion, and a gate electrode layeris provided to cross the protrusion in the longitudinal direction. Thegate electrode layer covers an upper surface and side surfaces of theprotrusion with a gate insulating film placed between the gate electrodelayer and the protrusion. With the second transistor having a fin-typestructure, the channel width can be reduced to achieve higherintegration of transistors. Moreover, a larger amount of current canflow through the transistor and the control efficiency can be increased,so that the off-state current and threshold voltage of the transistorcan be reduced.

Contact plugs 913 and 915 are connected to the impurity regions 906 inthe substrate 900. Here, the contact plugs 913 and 915 also function asa source electrode and a drain electrode of the transistor 901. Inaddition, impurity regions that are different from the impurity regions906 are provided between the impurity regions 906 and the channelformation region. The impurity regions function as LDD regions orextension regions for controlling the distribution of electric fields inthe vicinity of the channel formation region, depending on theconcentration of an impurity introduced thereto. Sidewall insulatingfilms 909 are provided at side surfaces of the gate electrode layer 908with an insulating film placed therebetween. By using this insulatingfilm and the sidewall insulating films 909, the LDD regions or extensionregions can be formed.

The transistor 901 is covered with an insulating film 910. Theinsulating film 910 can function as a protective film and can preventimpurities from entering the channel formation region from the outside.With the insulating film 910 formed by CVD using a material such assilicon nitride, hydrogenation can be performed by heat treatment in thecase where single crystal silicon is used for the channel formationregion. When an insulating film having tensile stress or compressivestress is used as the insulating film 910, distortion can be provided tothe semiconductor material used for the channel formation region. Byapplication of tensile stress to a silicon material used for the channelformation region of an n-channel transistor or application ofcompressive stress to a silicon material used for the channel formationregion of a p-channel transistor, the field-effect mobility of thetransistor can be increased.

An insulating film 911 is provided over the insulating film 910, and asurface of the insulating film 911 is planarized by CMP. Consequently,element layers can be stacked with high accuracy above a layer includingthe transistor 901.

A layer including the transistor 902 in which an oxide semiconductorfilm is used for a channel is formed above the layer including thetransistor 901. The transistor 902 is a top-gate transistor. Thetransistor 902 includes a source electrode layer 927 and a drainelectrode layer 928 in contact with side surfaces and an upper surfaceof an oxide semiconductor film 926, and includes a gate electrode layer930 over a gate insulating film 929 placed over the oxide semiconductorfilm 926, the source electrode layer 927, and the drain electrode layer928. Insulating films 932 and 933 are formed to cover the transistor902. Here, a method of fabricating the transistor 902 will be describedbelow.

The oxide semiconductor film 926 is formed over the insulating film 924.The insulating film 924 can be formed using an inorganic insulating filmof silicon oxide, silicon nitride, silicon nitride oxide, siliconoxynitride, aluminum oxide, aluminum nitride, aluminum nitride oxide, orthe like. In particular, the insulating film 924 is preferably formedusing a material with a low dielectric constant (a low-k material)because capacitance due to overlap of electrodes or wirings can besufficiently reduced. Note that the insulating film 924 may be a porousinsulating film containing any of the above materials. Since the porousinsulating film has lower dielectric constant than a dense insulatingfilm, parasitic capacitance due to electrodes or wirings can be furtherreduced. In this example, the insulating film 924 is a stack of asilicon oxide film with a thickness of about 300 nm on a 50-nm-thickaluminum oxide film.

The oxide semiconductor film 926 can be formed by processing an oxidesemiconductor film formed over the insulating film 924 into a desiredshape. The thickness of the oxide semiconductor film is 2 nm to 200 nm,preferably 3 nm to 50 nm, more preferably 3 nm to 20 nm. The oxidesemiconductor film is formed by sputtering using an oxide semiconductortarget. Moreover, the oxide semiconductor film can be formed bysputtering under a rare gas (e.g., argon) atmosphere, an oxygenatmosphere, or a mixed atmosphere of a rare gas (e.g., argon) andoxygen.

Note that before the oxide semiconductor film is formed by sputtering,dust on a surface of the insulating film 924 is preferably removed byreverse sputtering in which an argon gas is introduced and plasma isgenerated. The reverse sputtering refers to a method in which, withoutapplication of voltage to a target side, an RF power source is used forapplication of voltage to a substrate side in an argon atmosphere togenerate plasma in the vicinity of the substrate to modify a surface.Note that instead of an argon atmosphere, a nitrogen atmosphere, ahelium atmosphere, or the like may be used. Alternatively, an argonatmosphere to which oxygen, nitrous oxide, or the like is added may beused. Further alternatively, an argon atmosphere to which chlorine,carbon tetrafluoride, or the like is added may be used.

In this example, an In—Ga—Zn-based oxide semiconductor thin film thathas a thickness of 30 nm and is obtained by sputtering using a targetincluding indium (In), gallium (Ga), and zinc (Zn) is used as the oxidesemiconductor film. As the target, it is preferable to use a targethaving an atomic ratio of In:Ga:Zn=1:1:1, 4:2:3, 3:1:2, 1:1:2, 2:1:3, or3:1:4. The filling rate of the target including In, Ga, and Zn is higherthan or equal to 90% and lower than or equal to 100%, preferably higherthan or equal to 95% and lower than 100%. With the use of the targetwith high filling rate, a dense oxide semiconductor film is formed.

In the case where an In—Zn-based material is used for the oxidesemiconductor film, a target to be used has a composition ratio ofIn:Zn=50:1 to 1:2 in an atomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molarratio), preferably In:Zn=20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1to 1:2 in a molar ratio), more preferably In:Zn=15:1 to 1.5:1 in anatomic ratio (In₂O₃:ZnO=15:2 to 3:4 in a molar ratio). For example, in atarget used for formation of an In—Zn-based oxide semiconductor with anatomic ratio of In:Zn:O═X:Y:Z, the relation of Z>1.5X+Y is satisfied.The mobility can be improved by keeping the ratio of Zn within the aboverange.

In the case where an In—Sn—Zn-based oxide semiconductor film is formedas the oxide semiconductor film by sputtering, it is preferable to usean In—Sn—Zn—O target having an atomic ratio of In:Sn:Zn=1:1:1, 2:1:3,1:2:2, or 20:45:35.

In this example, the oxide semiconductor film is formed in such a mannerthat the substrate is held in a process chamber kept in a reducedpressure state, a sputtering gas from which hydrogen and moisture areremoved is introduced while moisture remaining in the process chamber isremoved, and the above-described target is used. The substratetemperature during the deposition may be 100° C. to 600° C., preferably200° C. to 400° C. By forming the oxide semiconductor film while thesubstrate is heated, the concentration of impurities included in theformed oxide semiconductor film can be reduced. In addition, damage bysputtering can be reduced. In order to remove remaining moisture in theprocess chamber, an entrapment vacuum pump is preferably used. Forexample, a cryopump, an ion pump, or a titanium sublimation pump ispreferably used. The evacuation unit may be a turbo pump provided with acold trap. In the process chamber which is evacuated with the cryopump,for example, a hydrogen atom, a compound containing a hydrogen atom,such as water (H₂O), and the like are removed, whereby the impurityconcentration in the oxide semiconductor film formed in the processchamber can be reduced.

As one example of the deposition conditions, the distance between thesubstrate and the target is 100 mm, the pressure is 0.6 Pa, thedirect-current (DC) power is 0.5 kW, and the atmosphere is an oxygenatmosphere (the proportion of the oxygen flow rate is 100%). Note that apulsed direct-current (DC) power source is preferably used because dustgenerated in deposition can be reduced and the film thickness can bemade uniform.

By setting the leakage rate of the process chamber of the sputteringapparatus to 1×10⁻¹⁰ Pa·m³/s or less, impurities, such as an alkalimetal or a hydride, which enter the oxide semiconductor film that isbeing formed by sputtering can be reduced. Further, with the use of anentrapment vacuum pump as an exhaustion system, counter flow ofimpurities such as an alkali metal, a hydrogen atom, a hydrogenmolecule, water, or a hydride from the exhaustion system can be reduced.

When the purity of the target is set to 99.99% or higher, an alkalimetal, a hydrogen atom, a hydrogen molecule, water, a hydroxyl group, ahydride, or the like entering the oxide semiconductor film can bereduced. In addition, when the above target is used, the concentrationof an alkali metal such as lithium, sodium, or potassium can be reducedin the oxide semiconductor film.

In order that the oxide semiconductor film contains hydrogen, a hydroxylgroup, and moisture as little as possible, it is preferable thatimpurities such as moisture or hydrogen that are adsorbed on thesubstrate 900 be desorbed and exhausted by preheating of the substrate900 over which the insulating film 924 is formed in a preheating chamberof a sputtering apparatus, as pretreatment for deposition. Thetemperature for the preheating is 100° C. to 400° C., preferably 150° C.to 300° C. As an evacuation unit provided in the preheating chamber, acryopump is preferably used. Note that this preheating treatment is notnecessarily performed.

Note that etching for forming the oxide semiconductor film 926 may bedry etching, wet etching, or both dry etching and wet etching. As anetching gas used for dry etching, it is preferable to use a gascontaining chlorine (a chlorine-based gas such as chlorine (Cl₂), borontrichloride (BCl₃), silicon tetrachloride (SiCl₄), or carbontetrachloride (CCl₄)). Alternatively, it is possible to use a gascontaining fluorine (a fluorine-based gas such as carbon tetrafluoride(CF₄), sulfur hexafluoride (SF₆), nitrogen trifluoride (NF₃), ortrifluoromethane (CHF₃)), hydrogen bromide (HBr), oxygen (O₂), any ofthese gases to which a rare gas such as helium (He) or argon (Ar) isadded, or the like.

As the dry etching method, a parallel plate reactive ion etching (RIE)method or an inductively coupled plasma (ICP) etching method can beused. In order to etch the film into a desired shape, the etchingconditions (e.g., the electric power applied to a coiled electrode, theelectric power applied to an electrode on the substrate side, and theelectrode temperature on the substrate side) are adjusted asappropriate.

A resist mask used for forming the oxide semiconductor film 926 may beformed by an inkjet method. Formation of the resist mask by an inkjetmethod needs no photomask; thus, fabrication cost can be reduced.

Note that it is preferable that reverse sputtering be performed beforethe formation of a conductive film in a subsequent step so that a resistresidue and the like that attach onto surfaces of the oxidesemiconductor film 926 and the insulating film 924 are removed.

Note that the oxide semiconductor film formed by sputtering or the likesometimes contains a large amount of moisture or hydrogen (including ahydroxyl group) as impurities. Moisture or hydrogen easily forms donorlevels and thus serves as impurities in the oxide semiconductor.Therefore, in this example of the present invention, in order to reduceimpurities such as moisture and hydrogen in the oxide semiconductor film(dehydrate or dehydrogenate the oxide semiconductor film), the oxidesemiconductor film 926 is subjected to heat treatment in areduced-pressure atmosphere, an inert gas atmosphere of nitrogen, a raregas, or the like, an oxygen gas atmosphere, or an ultra-dry airatmosphere (with a moisture content of 20 ppm (−55° C. by conversioninto a dew point) or less, preferably 1 ppm or less, more preferably 10ppb or less, in the case where the measurement is performed by a dewpoint meter of a cavity ring down laser spectroscopy (CRDS) system).

By performing heat treatment on the oxide semiconductor film 926,moisture or hydrogen in the oxide semiconductor film 926 can beeliminated. Specifically, heat treatment may be performed at atemperature higher than or equal to 250° C. and lower than or equal to750° C., preferably higher than or equal to 400° C. and lower than thestrain point of the substrate. For example, heat treatment may beperformed at 500° C. for approximately 3 minutes to 6 minutes. When RTAis used for the heat treatment, dehydration or dehydrogenation can beperformed in a short time; thus, treatment can be performed even at atemperature higher than the strain point of a glass substrate.

In this example, an electric furnace that is one of heat treatmentapparatuses is used.

Note that the heat treatment apparatus is not limited to an electricfurnace, and may have a device for heating an object by heat conductionor heat radiation from a heating element such as a resistance heatingelement. For example, a rapid thermal annealing (RTA) apparatus such asa gas rapid thermal annealing (GRTA) apparatus or a lamp rapid thermalannealing (LRTA) apparatus can be used. An LRTA apparatus is anapparatus for heating an object by radiation of light (anelectromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressuresodium lamp, or a high pressure mercury lamp. A GRTA apparatus is anapparatus for heat treatment using a high-temperature gas. As the gas,an inert gas which does not react with an object by heat treatment, likenitrogen or a rare gas such as argon, is used.

In the heat treatment, it is preferable that moisture, hydrogen, and thelike be not contained in nitrogen or a rare gas such as helium, neon, orargon. Alternatively, the purity of nitrogen or a rare gas such ashelium, neon, or argon which is introduced into the heat treatmentapparatus is preferably 6N (99.9999%) or higher, more preferably 7N(99.99999%) or higher (i.e., the impurity concentration is preferably 1ppm or lower, more preferably 0.1 ppm or lower).

Through the above-described steps, the concentration of hydrogen in theoxide semiconductor film 926 can be reduced and the oxide semiconductorfilm 926 can be highly purified. Thus, the oxide semiconductor film canbe stable. In addition, by using the oxide semiconductor film in whichthe hydrogen concentration is reduced and the purity is improved, it ispossible to fabricate a transistor with high withstand voltage andextremely small off-state current. The above heat treatment can beperformed at any time after the oxide semiconductor film is formed.

Next, the source electrode layer 927 and the drain electrode layer 928are formed by a photolithography process. Specifically, the sourceelectrode layer 927 and the drain electrode layer 928 can be formed insuch a manner that a conductive film is formed over the insulating film924 by sputtering or vacuum evaporation and then processed (patterned)into a predetermined shape.

In this example, a 100-nm-thick tungsten film is used for the sourceelectrode layer 927 and the drain electrode layer 928.

Note that the materials and etching conditions are adjusted asappropriate so that the oxide semiconductor film 926 is not removed asmuch as possible in etching of the conductive film. Depending on theetching conditions, an exposed portion of the oxide semiconductor film926 is partially etched and thus a groove (a depressed portion) isformed in some cases.

In this example, a tungsten film is used as the conductive film to bethe source electrode layer 927 and the drain electrode layer 928. Thus,wet etching can be selectively performed on the conductive film using asolution containing ammonia and hydrogen peroxide water (an ammoniahydrogen peroxide mixture). As the ammonia hydrogen peroxide mixture, asolution in which 31 wt % hydrogen peroxide water, 28 wt % ammoniawater, and water are mixed at a volume ratio of 5:2:2 is specificallyused. Alternatively, dry etching may be performed on the conductive filmwith the use of a gas containing carbon tetrafluoride (CF₄), chlorine(Cl₂), or oxygen.

In order to reduce the number of photomasks and steps in aphotolithography process, etching may be performed with the use of aresist mask formed using a multi-tone mask which is a light-exposuremask through which light is transmitted so as to have a plurality ofintensities. A resist mask formed using a multi-tone mask has aplurality of thicknesses and can be changed in shape by ashing; thus,the resist mask can be used in a plurality of etching steps forprocessing films into different patterns. That is, a resist maskcorresponding to at least two kinds of different patterns can be formedby one multi-tone mask. As a result, the number of light-exposure maskscan be reduced and the number of corresponding photolithographyprocesses can also be reduced, whereby the fabrication process can besimplified.

Further, an oxide conductive film functioning as a source region and adrain region may be provided between the oxide semiconductor film 926and the source and drain electrode layers 927 and 928. The material ofthe oxide conductive film preferably contains zinc oxide as a componentand preferably does not contain indium oxide. For such an oxideconductive film, zinc oxide, zinc aluminum oxide, zinc aluminumoxynitride, zinc gallium oxide, or the like can be used.

For example, in the case where the oxide conductive film is formed,patterning for forming the oxide conductive film and patterning forforming the source electrode layer 927 and the drain electrode layer 928may be performed concurrently.

By providing the oxide conductive film functioning as the source anddrain regions, the resistance between the oxide semiconductor film 926and the source and drain electrode layers 927 and 928 can be lowered, sothat the transistor can operate at high speed. In addition, by providingthe oxide conductive film functioning as the source and drain regions,the withstand voltage of the transistor can be increased.

Next, plasma treatment may be performed using a gas such as N₂O, N₂, orAr. By this plasma treatment, water or the like attached onto an exposedsurface of the oxide semiconductor film is removed. Plasma treatment maybe performed using a mixture gas of oxygen and argon.

After the plasma treatment, the gate insulating film 929 is formed tocover the source and drain electrode layers 927 and 928 and the oxidesemiconductor film 926. Then, over the gate insulating film 929, thegate electrode layer 930 is formed to overlap with the oxidesemiconductor film 926.

In this example, a 20-nm-thick silicon oxynitride film formed bysputtering is used as the gate insulating film 929. The substratetemperature during the film formation may range from room temperature to400° C. and is 300° C. in this example.

After the gate insulating film 929 is formed, heat treatment may beperformed. The heat treatment is performed in a nitrogen atmosphere,ultra-dry air, or a rare gas (e.g., argon or helium) atmospherepreferably at 200° C. to 400° C., and for example at 250° C. to 350° C.It is preferable that the water content in the gas be 20 ppm or less,preferably 1 ppm or less, more preferably 10 ppb or less. In thisexample, for example, heat treatment is performed in a nitrogenatmosphere at 250° C. for 1 hour. Alternatively, RTA treatment for ashort time at a high temperature may be performed before the formationof the source and drain electrode layers 927 and 928 in a manner similarto that of the heat treatment performed on the oxide semiconductor filmfor reduction of moisture or hydrogen. Even when oxygen vacancies aregenerated in the oxide semiconductor film 926 by the previous heattreatment performed on the oxide semiconductor film 926, oxygen issupplied to the oxide semiconductor film 926 from the gate insulatingfilm 929 by performing heat treatment after the gate insulating film 929containing oxygen is provided. By the supply of oxygen to the oxidesemiconductor film 926, oxygen vacancies that serve as donors can bereduced in the oxide semiconductor film 926 and the stoichiometriccomposition can be satisfied. As a result, the oxide semiconductor film926 can be made to be substantially i-type and variations in electricalcharacteristics of the transistor due to oxygen vacancies can bereduced; thus, electrical characteristics can be improved. There is noparticular limitation on the timing of this heat treatment as long as itis after the formation of the gate insulating film 929. When this heattreatment doubles as another step, the oxide semiconductor film 926 canbe made to be substantially i-type without increase in the number ofsteps.

Moreover, oxygen vacancies that serve as donors in the oxidesemiconductor film 926 may be reduced by subjecting the oxidesemiconductor film 926 to heat treatment in an oxygen atmosphere so thatoxygen is added to the oxide semiconductor. The heat treatment isperformed at a temperature, for example, higher than or equal to 100° C.and lower than 350° C., preferably higher than or equal to 150° C. andlower than 250° C. It is preferable that an oxygen gas used for the heattreatment in an oxygen atmosphere do not include water, hydrogen, or thelike. Alternatively, the purity of the oxygen gas which is introducedinto the heat treatment apparatus is preferably 6N (99.9999%) or higher,more preferably 7N (99.99999%) or higher (i.e., the impurityconcentration in the oxygen gas is preferably 1 ppm or lower, morepreferably 0.1 ppm or lower).

Alternatively, oxygen may be added to the oxide semiconductor film 926by ion implantation, ion doping, or the like to reduce oxygen vacanciesserving as donors. For example, oxygen that is made into a plasma statewith a microwave at 2.45 GHz may be added to the oxide semiconductorfilm 926.

The gate electrode layer 930 can be formed in such a manner that aconductive film is formed over the gate insulating film 929 and then ispatterned.

The thickness of the gate electrode layer 930 is 10 nm to 400 nm,preferably 100 nm to 300 nm. In this example, the gate electrode layer930 is formed in the following manner: a 135-nm-thick tungsten film isstacked over a 30-nm-thick tantalum nitride film by sputtering to form aconductive film for the gate electrode, and then, the conductive film isprocessed (patterned) into a desired shape by etching. Note that aresist mask may be formed by an inkjet method. Formation of the resistmask by an inkjet method needs no photomask; thus, fabrication cost canbe reduced.

Through the above steps, the transistor 902 is formed.

Note that the transistor 902 is described as a single-gate transistor;if necessary, it is possible to fabricate a multi-gate transistor thatincludes a plurality of gate electrodes electrically connected to eachother and thus includes a plurality of channel formation regions.

In the fabrication method described above, the source electrode layer927 and the drain electrode layer 928 are formed after the oxidesemiconductor film 926. Accordingly, as illustrated in FIG. 7, in thetransistor 902 obtained by the above-described method, the sourceelectrode layer 927 and the drain electrode layer 928 are formed overthe oxide semiconductor film 926. Alternatively, in the transistor 902,the source and drain electrode layers 927 and 928 may be formed belowthe oxide semiconductor film 926, that is, between the oxidesemiconductor film 926 and the insulating film 924.

Note that an insulating film in contact with the oxide semiconductorfilm 926 may be formed using an insulating material containing anelement that belongs to Group 13 and oxygen. Many of oxide semiconductormaterials contain a Group 13 element, and an insulating materialcontaining a Group 13 element is compatible with an oxide semiconductor.Thus, when an insulating material containing a Group 13 element is usedfor the insulating film in contact with the oxide semiconductor film,the state of the interface between the oxide semiconductor film and theinsulating film can be kept favorable.

An insulating material containing a Group 13 element is an insulatingmaterial containing one or more elements that belong to Group 13. As theinsulating material containing a Group 13 element, gallium oxide,aluminum oxide, aluminum gallium oxide, gallium aluminum oxide, and thelike are given. Here, the amount of aluminum is larger than that ofgallium in atomic percent in aluminum gallium oxide, whereas the amountof gallium is larger than or equal to that of aluminum in atomic percentin gallium aluminum oxide.

For example, in the case of forming an insulating film in contact withan oxide semiconductor film containing gallium, a material containinggallium oxide may be used for the insulating film, so that favorablecharacteristics can be kept at the interface between the oxidesemiconductor film and the insulating film. For example, the oxidesemiconductor film and an insulating film containing gallium oxide areprovided in contact with each other, so that pileup of hydrogen at theinterface between the oxide semiconductor film and the insulating filmcan be reduced. Note that a similar effect can be obtained in the casewhere an element in the same group as a constituent element of the oxidesemiconductor is used in the insulating film. For example, it iseffective to form an insulating film with the use of a materialcontaining aluminum oxide. Note that water is less likely to permeatealuminum oxide, and it is therefore preferable to use a materialcontaining aluminum oxide in terms of preventing entry of water to theoxide semiconductor film.

The insulating film which is in contact with the oxide semiconductorfilm 926 preferably contains oxygen in a proportion higher than thestoichiometric composition by heat treatment in an oxygen atmosphere,oxygen doping, or the like. “Oxygen doping” refers to addition of oxygeninto a bulk. Note that the term “bulk” is used in order to clarify thatoxygen is added not only to a surface of a thin film but also to theinside of the thin film. In addition, “oxygen doping” includes “oxygenplasma doping” in which oxygen which is made to be plasma is added to abulk. The oxygen doping may be performed by ion implantation or iondoping.

By oxygen doping, an insulating film that includes a region where theproportion of oxygen is higher than that in the stoichiometriccomposition can be formed. When the insulating film including such aregion is in contact with the oxide semiconductor film, excess oxygen inthe insulating film is supplied to the oxide semiconductor film, andoxygen vacancies in the oxide semiconductor film or at the interfacebetween the oxide semiconductor film and the insulating film arereduced. Thus, the oxide semiconductor film can be made to be i-type orsubstantially i-type.

Note that the insulating film including a region where the proportion ofoxygen is higher than that in the stoichiometric composition may be usedas either the insulating film placed above the oxide semiconductor film926 or the insulating film placed below the oxide semiconductor film 926of the insulating films in contact with the oxide semiconductor film926; however, it is preferable to use such an insulating film as both ofthe insulating films in contact with the oxide semiconductor film 926.The above-described effect can be enhanced with a structure in which theinsulating films each including a region where the proportion of oxygenis higher than that in the stoichiometric composition are used as theinsulating films in contact with the oxide semiconductor film 926 andpositioned above and below the oxide semiconductor film 926 so that theoxide semiconductor film 926 is sandwiched between the insulating films.

The constituent elements may be the same or different between theinsulating films placed above and below the oxide semiconductor film926. The insulating film in contact with the oxide semiconductor film926 may be a stack of insulating films each including a region where theproportion of oxygen is higher than that in the stoichiometriccomposition.

Note that in this example, the transistor 902 has a top-gate structure.The transistor 902 includes a backgate electrode layer 923. With thebackgate electrode layer, the transistor 902 can serve as a normally-offtransistor. For example, when the potential of the backgate electrodelayer 923 is set at GND or a fixed potential, the threshold voltage ofthe transistor 902 can shift further in a positive direction, whichleads to the formation of a normally-off transistor.

In order to electrically connect the transistor 901 and the transistor902 to form an electric circuit, one or more wiring layers forconnecting these elements are stacked between layers and on the upperlayer.

In FIG. 7, for example, one of the source and the drain of thetransistor 901 is electrically connected to a wiring layer 914 throughthe contact plug 913. The other of the source and the drain of thetransistor 901 is electrically connected to a wiring layer 916 throughthe contact plug 915. The gate of the transistor 901 is electricallyconnected to the drain electrode layer 928 of the transistor 902 througha contact plug 917, a wiring layer 918, a contact plug 921, a wiringlayer 922, and a contact plug 925.

The wiring layers 914, 916, 918, and 922, and the backgate electrodelayer 923 are embedded in insulating films. These wiring layers and thelike are preferably formed using a low-resistance conductive materialsuch as copper or aluminum. Alternatively, the wiring layers can beformed using graphene formed by CVD as a conductive material. Grapheneis a one-atom thick sheet of sp²-bonded carbon molecules or a stack of 2to 100 sheets of the carbon molecules. Examples of a method ofmanufacturing such graphene are thermal CVD by which graphene is formedon a metal catalyst; and plasma CVD by which graphene is formed frommethane, without using a catalyst, by plasma generated locally withultraviolet light irradiation.

By using such a low-resistance conductive material, RC delay of signalstransmitted through the wiring layers can be reduced. When copper isused for the wiring layers, a barrier film is formed in order to preventcopper from diffusing into the channel formation region. The barrierfilm can be a tantalum nitride film, a stack of a tantalum nitride filmand a tantalum film, a titanium nitride film, or a stack of a titaniumnitride film and a titanium film, for example, but is not limited to afilm containing such materials as long as the film has a function ofpreventing diffusion of a wiring material and has adhesion to the wiringmaterial, a base film, or the like. The barrier film may be formed as alayer that is independently formed, or may be formed in such a mannerthat a material of the barrier film is included in a wiring material andprecipitated by heat treatment on the inner wall of an opening providedin an insulating film.

The insulating films 911, 912, 919, 920, and 933 can be formed using aninsulator such as silicon oxide, silicon oxynitride, silicon nitrideoxide, borophosphosilicate glass (BPSG), phosphosilicate glass (PSG),silicon oxide to which carbon is added (SiOC), silicon oxide to whichfluorine is added (SiOF), tetraethylorthosilicate (TEOS) which issilicon oxide prepared from Si(OC₂H₅)₄, hydrogen silsesquioxane (HSQ),methyl silsesquioxane (MSQ), organosilicate glass (OSG), or an organicpolymer-based material. In particular, in the case of advancingminiaturization of a semiconductor device, parasitic capacitance betweenwirings is significant and signal delay is increased; therefore, therelative permittivity of silicon oxide (k=4.0 to 4.5) is too high, and amaterial with k=3.0 or less is preferably used. In addition, since CMPtreatment is performed after the wirings are embedded in the insulatingfilms, the insulating films need to have high mechanical strength. Theinsulating films can be made porous to have a lower dielectric constantas long as their mechanical strength can be secured. The insulatingfilms are formed by sputtering, CVD, a coating method including spincoating (also referred to as spin on glass (SOG)), or the like.

An insulating film functioning as an etching stopper for planarizationtreatment by CMP or the like that is performed after the wiring materialis embedded in the insulating films 911, 912, 919, 920, and 933 may beadditionally provided.

Each of the contact plugs 913, 915, 917, 921, and 925 is formed in sucha manner that an opening (a via hole) with a high aspect ratio is formedin the insulating film and is filled with a conductive material such astungsten. The opening is formed preferably by highly anisotropic dryetching and particularly preferably by reactive ion etching (RIE). Theinner wall of the opening is covered with a barrier film (diffusionprevention film) formed of a titanium film, a titanium nitride film, astack of such films, or the like, and a material such as tungsten orpolysilicon doped with phosphorus or the like fills the opening. Forexample, tungsten is embedded in the via hole by blanket CVD, and anupper surface of the contact plug is planarized by CMP.

Example 6

The semiconductor device of one embodiment of the present invention canbe used for electronic devices in a wide variety of fields, such asdigital signal processing, software-defined radio systems, avionicsystems (electronic devices used in aircraft, such as communicationsystems, navigation systems, autopilot systems, and flight managementsystems), ASIC prototyping, medical image processing, voice recognition,encryption, bioinformatics, emulators for mechanical systems, and radiotelescopes in radio astronomy.

Examples of consumer products among such electronic devices are displaydevices, personal computers, and image reproducing devices provided withrecording media (devices that reproduce the content of recording mediasuch as digital versatile discs (DVDs) and have displays for displayingthe reproduced images). Other examples of electronic devices that caninclude the semiconductor device of one embodiment of the presentinvention are mobile phones, game consoles including portable gameconsoles, portable information terminals, e-book readers, cameras suchas video cameras and digital still cameras, goggle-type displays (headmounted displays), navigation systems, audio reproducing devices (e.g.,car audio systems and digital audio players), copiers, facsimiles,printers, and multifunction printers. FIGS. 8A to 8F illustrate specificexamples of these electronic devices.

FIG. 8A illustrates a portable game console. The portable game consoleillustrated in FIG. 8A includes a housing 5001, a housing 5002, adisplay portion 5003, a display portion 5004, a microphone 5005, aspeaker 5006, an operation key 5007, a stylus 5008, and the like. Notethat although the portable game console illustrated in FIG. 8A has thetwo display portions 5003 and 5004, the number of display portionsincluded in the portable game console is not limited to two.

FIG. 8B illustrates a portable information terminal. The portableinformation terminal illustrated in FIG. 8B includes a first housing5601, a second housing 5602, a first display portion 5603, a seconddisplay portion 5604, a joint 5605, an operation key 5606, and the like.The first display portion 5603 is provided in the first housing 5601,and the second display portion 5604 is provided in the second housing5602. The first housing 5601 and the second housing 5602 are connectedto each other with the joint 5605, and the angle between the firsthousing 5601 and the second housing 5602 can be changed with the joint5605. Images on the first display portion 5603 may be switched dependingon the angle at the joint 5605 between the first housing 5601 and thesecond housing 5602. A display device with a position input function maybe used as at least one of the first display portion 5603 and the seconddisplay portion 5604. Note that the position input function can be addedby providing a touch panel in a display device. Alternatively, theposition input function can be added by providing a photoelectricconversion element called a photosensor in a pixel area of a displaydevice.

FIG. 8C illustrates a laptop personal computer. The laptop personalcomputer illustrated in FIG. 8C includes a housing 5401, a displayportion 5402, a keyboard 5403, a pointing device 5404, and the like.

FIG. 8D illustrates an electric refrigerator-freezer. The electricrefrigerator-freezer illustrated in FIG. 8D includes a housing 5301, arefrigerator door 5302, a freezer door 5303, and the like.

FIG. 8E illustrates a video camera. The video camera illustrated in FIG.8E includes a first housing 5801, a second housing 5802, a displayportion 5803, operation keys 5804, a lens 5805, a joint 5806, and thelike. The operation keys 5804 and the lens 5805 are provided in thefirst housing 5801, and the display portion 5803 is provided in thesecond housing 5802. The first housing 5801 and the second housing 5802are connected to each other with the joint 5806, and the angle betweenthe first housing 5801 and the second housing 5802 can be changed withthe joint 5806. Images displayed on the display portion 5803 may beswitched depending on the angle at the joint 5806 between the firsthousing 5801 and the second housing 5802.

FIG. 8F illustrates a passenger car. The passenger car illustrated inFIG. 8F includes a car body 5101, wheels 5102, a dashboard 5103, lights5104, and the like.

This application is based on Japanese Patent Application serial no.2012-152318 filed with Japan Patent Office on Jul. 6, 2012, and JapanesePatent Application serial no. 2013-004143 filed with Japan Patent Officeon Jan. 14, 2013, the entire contents of which are hereby incorporatedby reference.

1. A semiconductor device comprising: a first node; a second node; afirst transistor in which a channel is formed in an oxide semiconductorlayer; a second transistor; and a circuit for supplying a potentialcorresponding to 1 or 0 to the second node, wherein one of a source anda drain of the first transistor is electrically connected to the firstnode, wherein the other of the source and the drain of the firsttransistor is electrically connected to the second node, and wherein agate of the second transistor is electrically connected to the other ofthe source and the drain of the first transistor.
 2. The semiconductordevice according to claim 1, wherein the first node is included in anarithmetic portion, and wherein the second node is included in a datastoring portion.
 3. The semiconductor device according to claim 1,wherein the circuit comprises: a first switch; a second switch; aninverter; and a wiring supplying the potential corresponding to 1 or 0,wherein the first switch is electrically connected to the first node andthe one of the source and the drain of the first transistor, wherein thesecond switch is electrically connected to the wiring, the first switchand the one of the source and the drain of the first transistor, andwherein the inverter is electrically connected to the first switch andthe second switch.
 4. The semiconductor device according to claim 3,wherein the second switch is a transistor.
 5. The semiconductor deviceaccording to claim 1, wherein the circuit comprises; a third transistorin which a channel is formed in an oxide semiconductor layer; and awiring supplying the potential corresponding to 1 or 0, wherein one of asource and a drain of the third transistor is electrically connected tothe wiring, and wherein the other of the source and the drain of thethird transistor is electrically connected to the second node.
 6. Thesemiconductor device according to claim 1, wherein the first transistoris provided over the second transistor with an insulating layertherebetween, and wherein a channel of the second transistor is formedin a silicon layer.
 7. A semiconductor device comprising: a first node;a first transistor electrically connected to the first node, wherein achannel of the first transistor is formed in an oxide semiconductorlayer; a second node electrically connected to the first transistor; asecond transistor electrically connected to the second node; and acircuit for supplying a potential corresponding to 1 or 0 to the secondnode, wherein the first node is capable of holding the potentialcorresponding to 1 or 0 as data in a period during which a power sourcevoltage is supplied to the semiconductor device, wherein the second nodeis capable of holding the data regardless of whether the power sourcevoltage is supplied, wherein the first transistor is configured tocontrol whether to electrically connect the first node and the secondnode, and wherein an on/off state of the second transistor depends on apotential of the second node.
 8. The semiconductor device according toclaim 7, wherein the circuit comprises: a switch; and a wiring supplyingthe potential corresponding to 1 or 0, wherein the switch is configuredto electrically connect one of a source and a drain of the firsttransistor either to the first node or to the wiring, and wherein thepotential corresponding to 1 or 0 is supplied to the second node whenthe one of the source and the drain of the first transistor iselectrically connected to the wiring and the first transistor is on. 9.The semiconductor device according to claim 7, wherein the circuitcomprises: a third transistor in which a channel is formed in an oxidesemiconductor layer; and a wiring supplying the potential correspondingto 1 or 0, wherein the third transistor is configured to control whetherto electrically connect the second node and the wiring, and wherein thepotential corresponding to 1 or 0 is supplied to the second node whenthe third transistor is on.
 10. The semiconductor device according toclaim 7, wherein the first transistor is provided over the secondtransistor with an insulating layer therebetween, and wherein a channelof the second transistor is formed in a silicon layer.
 11. A drivingmethod of a semiconductor device comprising: the semiconductor devicecomprising: a first node; a first transistor electrically connected tothe first node; a second node electrically connected to the firsttransistor; and a second transistor electrically connected the secondnode, and the driving method comprising: a first step of supplying apotential corresponding to 1 or 0 to the second node; a second step ofturning on the first transistor after the first step to store data heldin the first node in the second node; a third step of stopping supply ofa power source voltage to the semiconductor device after the secondstep; a fourth step of restarting the supply of the power source voltageto the semiconductor device after the third step; and a fifth step ofrestoring the data stored in the second node in the first node dependingon an on/off state of the second transistor after the fourth step. 12.The driving method of the semiconductor device according to claim 11,wherein a length of a period during which the first step is performed islonger than a length of a period during which the second step isperformed.
 13. The driving method of the semiconductor device accordingto claim 11, wherein the semiconductor device further comprises: aswitch; and a wiring supplying the potential corresponding to 1 or 0,and wherein the first step is performed by electrically connecting thefirst transistor and the wiring by the switch.
 14. The driving method ofthe semiconductor device according to claim 11, wherein thesemiconductor device further comprises: a third transistor in which achannel is formed in an oxide semiconductor layer; and a wiringsupplying the potential corresponding to 1 or 0, and wherein the firststep is performed by electrically connecting the second node and thewiring by the third transistor.
 15. The driving method of thesemiconductor device according to claim 11, wherein a channel of thefirst transistor is formed in an oxide semiconductor layer.
 16. Thedriving method of the semiconductor device according to claim 11,wherein the first transistor is provided over the second transistor withan insulating layer therebetween, and wherein a channel of the secondtransistor is formed in a silicon layer.